The present invention relates to a method of forming a fluorescent screen for a field emission cathode-ray display by an electrodeposition process and, more particularly, to a method of forming a fluorescent screen provided with a uniform fluorescent coating for a field emission cathode-ray display by electrodeposition, capable of preventing the deposition of a fluorescent material in an ineffective area in the surface of the fluorescent screen.
Generally, the fluorescent films of the fluorescent screen of a cathode-ray tube, such as a color cathode-ray tube or a monochromatic cathode-ray tube, are formed principally by a slurry process, and the fluorescent films of the fluorescent screens of some cathode-ray tubes are formed by a printing process or an electrodeposition process. The fluorescent screen of a field effect display is provided with a plurality of pillars arranged in a pattern to secure a vacuum and it is therefore very difficult to form fluorescent films between the plurality of pillars on the screen panel by a slurry process or a printing process. However, fluorescent films can be formed in desired areas corresponding to an electrode pattern on a screen panel by an electrodeposition process even if the screen panel has pillars. A method of forming fluorescent films on a screen panel by an electrodeposition process is proposed in Japanese Patent Application No. 4-225994.
In an electrodeposition process, a screen panel provided with transparent electrodes, and a counter electrode are immersed opposite to each other in an electrodeposition solution prepared by dispersing particles of a fluorescent material in an aqueous or nonaqueous electrolytic solution containing an electrolyte for positively or negatively charging the particles of the fluorescent material, a negative (positive) voltage and a positive (negative) voltage are applied respectively to the the transparent electrodes of the screen panel and the counter electrode when the fluorescent material is positively (negatively) charged to deposit the fluorescent material over the surfaces of the electrodes.
A fluorescent screen fabricating method disclosed in Japanese Patent Publication (Kokoku) No. 60-11415 for forming a color fluorescent screen provided with green fluorescent stripes, blue fluorescent stripes and red fluorescent stripes repeats the electrodeposition process to form the green fluorescent stripes, the blue fluorescent stripes and the red fluorescent stripes over transparent electrode stripes. When the conventional electrodeposition process employing a solid transparent electrode for forming a monochromatic fluorescent screen is employed in forming a fluorescent screen, the electric field is concentrated on the edges of a pattern, portions of a fluorescent film covering the edge areas of the pattern are formed in a thickness greater than a portion of the same covering the central area of the pattern, the fluorescent film extends beyond the edges of the pattern and hence the fluorescent film cannot uniformly be formed on the screen panel. When the conventional electrodeposition process is applied to forming fluorescent stripes on transparent stripe electrodes, the electric field is concentrated on areas near the lower end, the left end and the right end of a pattern of the transparent stripe electrodes and, consequently, portions of the fluorescent films covering those areas are formed in a thickness greater than that of other portions covering other areas of the pattern, the fluorescent stripes extend beyond the edges of the stripe electrodes and hence the fluorescent stripes cannot uniformly be formed on the pattern of the transparent stripe electrodes. Furthermore, when either the transparent solid electrode or the stripe electrodes are coated with the fluorescent films by the electrodeposition process, the fluorescent material is attracted to an ineffective area, i.e., an area in which the fluorescent material need not be deposited, by nonelectro-static attraction, such as van der Waals force and, when electrons impinge on the ineffective or inactive area, the fluorescent materials coating the ineffective or inactive area are excited and become luminous to deteriorate the picture quality of the fluorescent screen.